The present invention pertains to exploration electrodes and, more particularly, to exploration electrodes used in magnetotelluric measurements which determine the resistivity of sub-surface earth formations.
Presently, extensive geophysical and geological exploration is done particularly in the oil industry prior to drilling a producing well. Exploration is done from the initial stages to the final stage, which increase in cost accordingly. Although seismic exploration is the primary tool and is frequently performed prior to expensive well drilling, initial exploration tools are available for use in determining whether the expense of seismic exploration is justified in any potentially oil-bearing area. An initial exploration tool is a magnetotelluric survey, which measures the resistivity of formations in a given area to determine whether the resistances measured indicate a suitable sub-surface geology, depending upon the exploration objective.
A method for taking magnetotelluric measurements is to place four electrodes in a pattern defining a square with each electrode in a corner. Of the four electrodes, opposite corners of the square are paired. Electrical impulses are received from natural electrical energy in the earth from one electrode to the other electrode of the pair. For complete magnetotelluric survey information an electromagnetic coil is needed to measure the magnetic effects of the earth at the location being surveyed. The electrode pairs give the E components of the survey, normally designated as E.sub.x and E.sub.y. The E measurements are correlated with H measurements (H.sub.x and H.sub.y) which are detected by the electromagnetic coil. Resolution of the exact point for which the resistivity is being measured at a given depth deteriorates as a greater depth is surveyed. The frequency of the electrical waves indicates the depth of the formation for which the resistance is measured. For example, the resistance of a shallow sub-surface formation can be measured by detecting high frequency electromagnetic waves. To obtain the resistance of deeper formations, lower frequencies of the telluric waves are measured. The depth of the formation having the measured resistivity is calculated by combining the telluric electrical energy and the magnetic field measured. Due to the deterioration of the telluric signal, lower frequencies measuring the resistivity of deeper formations will indicate a resistivity that may be the resistivity of an infinite number of points on a common depth plane. For a more detailed discussion of magnetotelluric surveying and electrode placement techniques, reference is made to a pending application "Multiple Site Magnetotelluric Measurements" Ser. No. 63,491 filed by Marvin A. Bloomquist, et al, assigned to the same assignee as the present application.
The first type of telluric electrical survey electrodes were glass containers filled with cadmium chloride. These electrodes provided problems in deployment and retrieval along with their potential toxicity if broken. Initially, more durable electrodes made from a conductive metal such as lead were considered undesirable due to the noise associated with lead electrodes. However, once the outer layer of a lead electrode oxidizes, the noise is greatly reduced.
Present telluric electrodes are normally a square plate constructed of lead having a single conductor affixed thereto connecting the electrode to an amplifier. As illustrated in FIGS. 1 and 2, the electrode is normally placed in a hole manually dug into the ground. This hole is typically one to two feet deep. On top of the electrode is normally placed a mud slurry, as shown at 10 in FIG. 2. This deployment procedure has many disadvantages, one of which is the cost associated with manually or mechanically digging a hole large enough to place the flat lead plate electrode. However, as great a problem as deployment of telluric electrodes may be, retrieval presents a greater problem. To retrieve the flat plate electrode E, the hole must be redug, which may result in damage to the lead plate from contact with the shovel. Even though the mud covering the electrode is generally in liquid form, the electrode cannot be merely pulled out of the ground due to its configuration (See FIG. 2). Conductor 8 merely pulls out of electrode E. Since a hole must be dug for each electrode, the time and cost required for deployment and retrieval of a line of electrodes is proportional as the length of the line increases to the number of sites in the survey.